class Solution
{
public:
    bool hasCycle(ListNode* head)
    {
        if (head == nullptr) return false;

        ListNode* slow = head;
        ListNode* fast = head->next;
        while (fast != nullptr && fast->next != nullptr)
        {
            if (slow == fast) return true;

            slow = slow->next;
            fast = fast->next->next;
        }
        return false;
    }
};